// keys:用于从数组创建一个包含数组键的可迭代对象。
let fruits = ["Banana", "Orange", "Apple", "Mango"];

let res = fruits.keys();
for (let i = 0; i < fruits.length; i++) {
  console.log(res.next());
  /* 
    { value: 0, done: false }
    { value: 1, done: false }
    { value: 2, done: false }
    { value: 3, done: false }
  */
}
console.log(res.next(), fruits);
